How easy or not it is to maintain isn't what punishing means.
Burd sumarized it way better than I could though: It isn't that you can lose GL that makes it punishing, plenty of jobs have that. It is because the options for Monk to recover GL are really bad (and they need to be because its sorta part of the identity of GL compared to every other job buff). Losing GL is about as intense a punishment as losing Enochian (about 40% of your DPS), but losing Enochian may affect you for as little as one cast if you didn't make too many mistakes in the fight: You can recover from whatever mechanic sent you to scatter and get right back into it with full potency. Meanwhile, GL is out 40% of its DPS for about 15-20 seconds no matter what you do, and you need to maintain uptime to get it back. Not to mention MNK has an actual complicated rotation while BLM is basically 4 buttons. There is just a really good reason people don't whine about Enochian (which is nearly identical to GL besides the fact it forgives you once every 30 seconds and actively downgrades your rotation to really tell you that you mucked up) and GL.
PLG is a starter job, and I suspect the reason they nuked it so fast is because the sheer willingness MNK has to dumpster you with no recourse (even if it is in theory easy to avoid) probably isn't great for retention. The decision to gut Monk without a plan to replace GL with anything was a pretty extreme one, and that speaks pretty heavily to it not being a 'forum dweller bellyacher' problem and way more a 'We are noticing a LOT of players who started as PLG quitting without subbing by 50 on the Free Trial' problem.